NATURAL REPRODUCTION OF TROUT

All too little is known at present
about the relative importance of natural reproduction in maintaining fish
populations in the lakes and
streams. It is known, however, that
in many of the less accessible areas
of the park, natural spawning alone
has been responsible for maintaining the trout population after initial
stocking. Thus the planting of trout
is only warranted in waters which
show that the natural reproduction is
not capable of replenishing the supply of fish removed by anglers or
lost through other causes.

The trout of our mountain areas
such as the Sierra Nevada are resident; that is they do not make long
migrations to spawning grounds as
do salmon and trout of many coastal
streams. An interesting thing to note
is the fact that the salmon spawn
once and die, while trout may spawn
for several successive seasons.

The time of spawning among all
trout varies with the temperature,
season, locality and strain of fish.
In Yosemite, the rainbow and golden
trout spawn with rising water temperatures from early spring to midsummer
depending upon the elevation while brook and brown trout
sprawn from late summer to late
fall when water temperatures are
falling.

The spawning trout tend to move
upstream and seek the smaller tributaries,
or in the case of those inhabiting lakes, inlets may be utilized.
(Outlets and occasionally
gravelly lake shores may be used
if there is no suitable inlet.)

Eggs of trout a;e laid in nest-like
depressions known as “redds,” built
by the female in gravel bottomed
areas, where currents are fairly
swift. In choosing a spawning place,
a permanent water supply of even
temperature is the most important
factor. Digging of the nest is accomplished by vigorous head and tail
movements of the female as she lies
on her side. The male usually
spends his time during this period
pugnaciously driving off other males
from the nesting area.

When the nest is prepared, both
the male and female occupy a position directly above it. At the same
moment that the eggs are deposited,
sperm or milt is discharged over
them by the male. The then fertilized
eggs fall into the pit of the nest after
which the female covers them by
further digging movements that stir
up the surrounding gravel. As indicated elsewhere, a high degree of
efficiency results from this simple
method of fertilization as almost all
the eggs become fertile. One female
may dig nests and deposit eggs several times within one season’s
spawning period. All parental care
of the offspring ceases with the covering of the nest.

Before they hatch, the eggs remain in the gravel for several weeks
or months depending upon the
species and environmental factors.
The newly hatched trout are known
as “fry” and have a portion of the
egg sac still attached. It is from the
egg sac that the fry receives its first
food. Shortly the sac is absorbed and
the fingerling wiggles forth from the
gravel and begins to feed upon minute aquatic organisms.